For gemini I use either castor[0] or dragonstone[1]. Both are quite good, dragonstone even has tabs.
Both are written for GTK, so they integrate quite good with GNOME.
Both are written for GTK, so they integrate quite good with GNOME.
They have done pretty much the opposite - they added a proper API of theming instead of having a stylesheet override. GTK didn't really have themes before, so this is quite nice.
That's not theming, that's just changing accent colors in the same theme. If I don't like dark background color, I can't change it. If I don't like the light background color, I can't change that either. All I can do is change accent colors.
The entire GTK and GNOME ecosystem is heading towards Android like customization and theming, which is basically non-existent at this point.
What?
GTK1 and GTK2 had zillions of actual themes and engines.
Now, there's just a re-coloring API where I can't change the dark background if I don't like it and that's "real" theming.
Of course, you could use it in KDE or MATE or anywhere else but it would look completely alien with different themes and, potentially, different fonts and icons. You can't set your own theme without resorting to hacks.
If someone has come across a GTK4 app which isn't restricted to a specific desktop environment, let me know. Meanwhile, I won't use any GTK4 apps in existence.